The invention concerns an equipment and a method in a twin-wire former of a board machine or of a paper machine in view of controlling the edge effect.
In gap formers, the web is formed by, out of the headbox, feeding a fibrous stock into a gap formed by two wires. The gap formed by the wires is substantially wider than the slice jet discharged from the headbox. When the jet arrives in the gap, the wires are placed apart from each other in the area of the jet, but the wires tend to enter into contact with each other as the draining proceeds in the lateral areas. In the lateral area, in the cross direction, the thickness of the jet does not go to zero steeply, but gently (FIG. 1B), When the web is being formed, water is drained out of it by the effect of pressure p=T/R. In the initial stage, when a fibrous mat has not yet been formed to a significant extent, the rate of draining of water is considerable. In this connection, the draining of water takes place in the direction of thickness of the web. When a fibrous mat is formed, the rate of draining becomes slower. In such a case, the flow in the lateral area of the jet tends to turn towards the side owing to the reduction of the cross-directional face defined by the wires. The flow can take place either inwards or outwards. As a result of this, great divergencies from a machine-direction fibre orientation can be seen clearly in the lateral area. This comes out in particular when large slice openings and poorly drainable stock grades are used.
The present invention concerns a method for control and elimination of the detrimental edge effect in a twin-wire web former and in particular in a gap former. The method can also be used for regulation of orientation angles and basis weights in the lateral areas of the stock web, and, thus, it is also suitable for a method of regulation of the cross-direction profiles of the web.
In Fourdrinier machines, at the edges of the web, deckle boards and edge raising means are employed in order that the stock suspension should not flow over the edge of the wire. In gap formers, such devices for the lateral areas have not been needed, because, typically, small slice openings and/or quickly draining stocks have been used. But the problem of the lateral area is also present in gap formers. It is a feature common of Fourdrinier and gap concepts that, when the slice openings at the headbox become larger, the problems in the lateral areas are emphasized.
Figure 1A is a side view of the slice jet and the initial draining area in a gap former. The covering wire has been illustrated to be straight and tangential to both rolls in the figure, and right at the edge of the wire, where there is no stock web between the wires, the situation is indeed similar to what is shown. FIG. 1B is a sectional view taken along the line Ixe2x80x94I in FIG. 1A. It is seen from the figure that at the edges the covering wire and the lower wire are in contact with one another. In the middle area of the stock web, the covering wire and the lower wire are placed apart from one another, because the web is between them. Between the middle area and the wire edges, there is an area in which the wires approach each other. The edge of the stock web is placed in this area.
Since the. edge effect described above is emphasized in the area in which the draining of water and the web formation have proceeded rather far, it affects the properties of the fibre mat that is formed. At the edges of the web the basis weight is lower than in the middle of the web. Also, the orientation angles are different in the lateral areas of the web, as compared with the middle. True enough, the edges of the web are trimmed off in a later stage of the process, but, when a large slice opening is used, the effects of the edge effect extend to a portion of the web that is not trimmed off. On the other hand, the smaller the portion that is trimmed off, the more cost- efficient can the process be made.
As a solution for the problem described above, a method is described by whose means the covering wire is raised apart from the lower wire, and, at the same time, carriage of the stock suspension away from between the wires over the web edges is prevented.
In accordance with the invention, in view of minimizing and controlling the effects of the edge effect, in the present patent application, it is suggested that, in the twin-wire area, an arrangement of equipment in particular connected with the former roll be employed, by means of which arrangement the point of coming together of the forming wires in the edge area is transferred further on the former roll, i.e. in the present invention the area of draining is extended in which the webs are still placed apart from one another. Thus, when the wires reach contact with each other, the web has already had time to be drained enough in the edge area, and, thus, no problems of fibre orientation or basis weight occur.
In the present patent application, it is suggested that such an arrangement of equipment be employed in which a rib extending to the lateral area of the web is used. The position of the rib can be regulated by means of adjustment devices both in the machine direction and in the cross direction of the machine. In this way it is possible to regulate the position of the rib in relation to the former roll, and, thus, it is possible to regulate a controlled reaching of contact between the wires.